1. Field of the Invention
The present invention relates to a thermosensitive recording material which develops an image successfully toned to black and exhibits excellent image preservability, and which shows an improved heat response (sensitivity) while effectively inhibiting the discoloration of background caused by exposure to light.
2. Related Art
Thermosensitive recording materials generally comprise a substrate and, provided thereon, a thermosensitive recording layer comprising an electron-donative colorless dye precursor and an electron-accepting color developer as the main components; the colorless dye precursor and the color developer react instantaneously with each other when heated with a thermal head, thermal pen, laser beam, etc. to develop an image. Such materials are disclosed, for example, in GB 1135540-A, CA 835302A and the like.
Such thermosensitive recording materials have advantages in that recording can be performed with a relatively simple device which is easy to maintain and free from noise. Accordingly, they are in use over a wide field of applications including recorders, facsimiles, printers, computer terminals, labels, and automatic ticket vendors.
While the thermosensitive recording materials using an electron-donative colorless dye precursor and an electron-accepting color developer mentioned above have favorable characteristics including good appearance, good feel, high image density and availability of developed colors of various hues, they have a drawback that preservability of record is poor; that is, the thermally developed color (i.e. record) becomes erased by reaction with a plasticizer or additive contained in a stationery article made of plastics such as polyvinyl chloride, or disappears readily when the record is brought contact with chemicals contained in food or cosmetics, or when the record is exposed to sunlight even for a short period of time. Due to this drawback, the thermosensitive recording materials sustain currently a certain limitation as to their uses, and improvement in record stability or preservance is eagerly desired.
As to thermosensitive recording materials in which, two components react upon heating to develop an image with good preservability, there are disclosed, for example, thermosensitive recording materials comprising an imino compound and an isocyanate compound as the two components in Japanese Patent Application Kokai (Laid-open) Nos. 58-54,085, 58-104,959, 58-149,388, 59-115,887 and 59-115,888, and U.S. Pat. No. 4,521,793.
While these thermosensitive recording materials develop images of good preservability, the developed image is sepia (or dark reddish-brown) in color which is less favored than black as a color of records. With the aim of developing the hue as black as possible, the present inventors disclosed, in the specification of U.S. Ser. No. 07/409,580, a thermosensitive recording material comprising three components of an aromatic isocyanate, an imino compound and a green-colored fluoran compound. As the result of addition of the green-colored fluoran compounds, the hue of the developed color approached black. However, it became apparent as a problem that the background of the recording material tended to yellow or lightly reddish when exposed to light. Moreover, heat response of the material was too poor to obtain recording images of sufficient density on a high speed printer unit like a group III facsimile.